In the communications field, as a baud rate of a system increases, bandwidth occupied by a single carrier is increasingly large. Therefore, when bandwidth of hardware of a transmitter and a receiver is insufficient, a signal loss is caused when a signal passes through the transmitter or the receiver, thereby affecting overall performance of the system. For a wavelength division multiplexing system, a signal is generally carried on multiple single carriers with fixed spacing. When bandwidth of a single carrier increases, crosstalk between channels is intensified, and system performance deteriorates.